On the seasonal dependence of relativistic electron fluxes

The nature of the seasonal dependence of relativistic electron fluxes in the Earth's outer zone is investigated using 11 years of data from sensors onboard the SAMPEX spacecraft. It is found that, the relativistic electron fluxes show a strong semiannual modulation. However, the highest electron fluxes occur at times well away from the nominal equinoxes, lagging them by about 30 days. The time lag also shows a solar cycle phase dependence for the peak fluxes. The electron peak fluxes lag the vernal equinox by almost 60 days during the ascending phase of the solar cycle while the time lag near the autumnal equinox remains unchanged. The observed times of the peak electron fluxes during the descending phase of the solar cycle agrees most closely with the Russel-Mcpherron effect and less so with the equinoctial effect even after including propagation effects for finite solar wind speed. The observed times of the electron peaks are in disagreement with the axial effect. The asymmetrical response of the relativistic electrons during the ascending part of the solar cycle remains a puzzle.